Quantum Computing: A Quick Primer

The big surprise at this year's Ignite was Microsoft's announcement that it is working on a quantum computer, and the revelation of a prototype. Following that announcement, I overheard some people saying that they didn't really understand what a quantum computer was. In some ways, I felt like I had a responsibility to clarify things by writing a post on the subject. At the same time, however, I seriously considered not writing this post because I wasn't sure that I could explain quantum computing in a meaningful way in such a short format, but here goes.

The first rule for understanding quantum computing is to forget everything that you know about the way that normal computers work. Normal computers use microscopic transistors to process data. Those transistors allow for binary operations. Think of a transistor as being like a light switch that can either be turned on or off. A switch that is in the on position might have a binary value of 1, while a switch in the off position would have a binary value of 0.

Quantum computers do not use binary values, nor do they use transistors (at least not in the traditional sense). Instead, quantum computers work by altering the state of subatomic particles, and using those particle states to reflect a data value. The reason why quantum computers do not use binary values is because the particles used to retain data can have more than two states.

To borrow (but modify) an analogy from Scientific American, think of a traditional computer's transistor based operations in terms of flipping a coin. Perhaps heads would reflect a binary value of 1, and tails would reflect a binary value of 0. Every time that you flip the coin, one of these binary values will be displayed.

But what if you could take away gravity and bend a few laws of physics? If that were the case, then our binary coin could theoretically display other values. If the coin landed on its edge, for example, then it would display neither heads nor tails. Instead, the value would be 50 percent heads and 50 percent tails. This is the very essence of quantum computing.

Quantum computing does not use bits like a normal computer. Instead it is based on the use of quantum bits, or qubits. I will spare you the explanation, but quantum mechanics make it very difficult to determine the value of a qubit. Measuring the value essentially causes the qubit to be destroyed before the value can be ascertained. What you can do, however, is to guess the value based on probabilities.

Remember the flipped coin analogy? What happens if the coin (which is still not bound by gravity or by the laws of physics in this example) were to land in a position that is almost, but not quite flat on the table (the coin is balanced at a slight incline). In other words, one of the coin's edges is touching the table, while the other edge is not. In such a case (however unlikely it may be) we could estimate that the coin's value is 80 percent heads and 20 percent tails. If we wanted to treat this value as binary, we could say that because the coin is mostly heads, then we will treat it as heads. Of course quantum computers are not based on binary values, and treating values as binary could undermine the benefits of quantum computing.

So without getting overly complex, the difference between binary and quantum is that in a binary system, each bit can hold a single piece of information -- 0 or 1. In a quantum system, each qubit can reflect a wide variety of states. To truly put this into prospective, consider that a 100-bit system would yield exactly 100 bits of information. That's just over a dozen bytes of data. In contrast, the Scientific American article that I referenced earlier indicates that a system with 100 qubits can reflect 1,267,650,600,228,229,401,496,703,205,375 individual pieces of information. That's a lot of data!

The important takeaway is that it is not raw capacity that makes quantum computing so powerful, but rather the way that the qubits are related to one another. A quantum computer can theoretically perform massive calculations in a single operation because of the way that the parallel qubits of data are related to one another. It is this parallel processing that makes quantum computers so unbelievably powerful.

About the Author

Brien Posey is a 16-time Microsoft MVP with decades of IT experience. As a freelance writer, Posey has written thousands of articles and contributed to several dozen books on a wide variety of IT topics. Prior to going freelance, Posey was a CIO for a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the country's largest insurance companies and for the Department of Defense at Fort Knox. In addition to his continued work in IT, Posey has spent the last several years actively training as a commercial scientist-astronaut candidate in preparation to fly on a mission to study polar mesospheric clouds from space. You can follow his spaceflight training on his Web site.